A technique for forming a transistor using a semiconductor layer formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)) has been attracting attention. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). Semiconductor materials typified by silicon are widely known as materials for semiconductor layers that can be used for transistors. As other materials, oxide semiconductors have been attracting attention.
For example, a technique for forming a transistor using an amorphous oxide containing In, Zn, Ga, Sn, and the like as an oxide semiconductor is disclosed (see Patent Document 1). A technique for forming a self-aligned top-gate transistor using an oxide layer is also disclosed (see Patent Document 2). In addition, a technique for forming a transistor in which an oxide layer where a channel is formed is electrically surrounded by electric fields of top and bottom gate electrodes to achieve high field-effect mobility is disclosed (see Patent Document 3).
In addition, a technique for forming a transistor with high electrical reliability, e.g., a small shift in threshold voltage by using an insulating layer that releases oxygen by being heated as a base insulating layer of an oxide semiconductor layer where a channel is formed, to reduce oxygen vacancies in the oxide semiconductor layer is disclosed (see Patent Document 4).